Vascular endothelial growth factor (VEGF) is a potent angiogenic factor for tumor angiogenesis and growth. We previously established the immunoneutralizing monoclonal antibodies (MAbs) to human VEGF, and showed that MV101 (IgG1) and MV303 (IgG2a) inhibited the growth of human solid tumor xenografts in nude mice. Then, we tried to develop another immunoneutralizing anti-VEGF MAb that exhibited more potent antitumor activity than MV101 or MV303. We obtained more than 140 clones of hybridomas that were producing anti-VEGF MAb from the mice immunized with recombinant human VEGF121. Among them, 26 clones showed the immunoneutralizing activity and MV833 possessed the most potent antitumor activity in vivo. A total of 9 i.p. administrations of 25 microg of MV833 inhibited the growth of human fibrosarcoma HT-1080 solid tumor xenografted in nude mice more potently than MV101 or MV303. Moreover, only 1 i.v. administration of 100 microg of MV833 on Day 1 after tumor inoculation also significantly inhibited the growth of HT-1080 in vivo, whereas MV101 and MV303 did not. All three MAbs inhibited the growth of human umbilical vein endothelial cells (HUVEC) induced by VEGF121 and the binding of 125I-labeled VEGF121 to HUVEC to a similar extent. The binding of MV101 and MV303 to VEGF121 was cross-competitive; however, MV833 weakly competed with the binding of MV101 to VEGF121. These findings indicated that MV833 recognized the region(s) of VEGF differently than MV101 or MV303, and this difference contributed to the superiority of antitumor activity of MV833.